Burlap and black. Can fill in with pops of color that could change with the seasons.
Twinkl Resources >> Message in a Bottle Writing Frames >> Printable resources for Primary, EYFS, KS1 and SEN. Thousands of classroom displays and teaching aids! Topics, Pirates, Writing Frames, Writing, Literacy, English
DIY Pirate Sign on a Budget - Even if you don't want to follow all the steps in the article, you could do something similar on the wall with paper. Either way it'd be a great addition to the pirate classroom theme!